This invention relates to dielectrics for electronic components based on mixtures of known liquid crystalline compounds, the viscosity of which is favorably affected by inclusion therein of certain biphenyl compounds.
Nematic or nematic-cholesteric liquid crystalline materials, which undergo significant changes in optical properties, such as light scattering, birefringence, reflecting power or color, under the influence of electric fields, are being used, to an increasing extent, as electro-optical indicator elements. The operation of such indicator elements depends, for example, on dynamic scattering, deformation of aligned phases or the Schadt-Helfrich effect in the twisted cell.
Liquid crystalline materials for commercial application in electronic indicator components, e.g., liquid crystal display elements, must satisfy a plurality of requirements. Especially important are:
(A) CHEMICAL STABILITY TO MOISTURE, AIR AND PHYSICAL INFLUENCES, SUCH AS HEAT, INFRARED, VISIBLE AND ULTRAVIOLET RADIATION AND DIRECT AND ALTERNATING ELECTRICAL FIELDS;
(B) A LIQUID CRYSTALLINE MESOPHASE IN THE TEMPERATURE RANGE OF +10.degree. C. to +50.degree. C., preferably from 0.degree. C. to 60.degree. C.;
(c) low viscosity at room temperature, preferably not above 70 cP; and
(D) NO INHERENT ABSORPTION OF VISIBLE LIGHT, THAT IS, THE COMPOUNDS MUST BE COLORLESS.
Numerous liquid crystalline substances satisfy the stability requirements for dielectrics in electronic constructional elements and are also colorless or have inherent color, which is substantially suppressed by suitable additives. See published German Patent Specification No. 2,209,127. p,p'-Disubstituted azoxybenzenes described in German Patent Specification No. 2,014,989 (U.S. Pat. No. 3,773,747), p,p'-disubstituted benzoic acid phenyl esters described in published German Patent Specification No. 2,139,628 (Ser. No. 277,502, filed Aug. 3, 1972), and p,p'-disubstituted biphenyl derivatives described in published German Patent Specification No. 2,356,085, are typical types of compounds, all of whose disclosures are incorporated by reference.
In these and other known series of compounds having a liquid crystalline mesophase, there are no individual compounds which form a liquid crystalline nematic mesophase in the required temperature range of 10.degree. C. to 50.degree. C. It is therefore conventional to prepare mixtures of two or more compounds to obtain materials usable as liquid crystalline dielectrics. Usually, a compound with a low melting and clear point is mixed with another having a markedly higher melting and clear point. Usually, the melting point of the mixture is below that of the lower melting component and the clear point is between the clear points of the components. However, optimum dielectrics cannot be prepared in this manner because the component having the high melting and clear point almost always imparts high viscosity to the mixture, so that switch times of electro-optical indicator elements produced therewith become undesirably long.
It is an object of this invention to provide liquid crystalline dielectrics which exhibit a nematic phase in the required temperature range and have sufficiently short switch times in liquid crystal cells at room temperature.